Constant force curl springs have been used in window balance systems where they have the advantage of applying a constant lifting force to counterbalance the constant weight of a window sash. The constant force of these springs is derived from the curling tendency of an uncurled length of a spring steel strip that has been formed to curl up. When the strips are uncurled and extended, each increment of the extended strip is biased to recurl itself and thus exerts a constant force against spring extension.
Curl springs have never been popular in window counterbalance systems, though, because each of their known arrangements have suffered from at least one competitive drawback. For example, sash mounted arrangements of curl springs have not allowed the sash to tilt; jamb mounted arrangements have taken up window space that manufacturers have been unwilling to commit to balance systems; and tilt sash arrangements have been inefficient and sometimes short-lived or inadequate in performance. The result is that only a few of the many different proposed arrangements of curl spring balance systems are presently marketed, and these have only a small market share.